1. Field of the Invention
The present invention relates to a superconductive rotating electric machine drive control system and a superconductive rotating electric machine drive control method to be used mainly in ships required to have low-speed large torque, as well as in wind-power generation, railroads, automobiles, power generation and industrial auxiliary machines, and the likes.
2. Related Art
Recently, attention is drawn to ships called “super-eco ships” having electric propulsion systems of environmental load reducing types. Those are ships of a new concept that achieve a high degree of freedom in placement by employing novel vessel shapes, contra-rotating propellers, and electric propulsion systems. By virtue of those techniques, the “super-eco ships” are being widely used.
FIG. 5 is a block diagram showing the structure of an electric propulsion system provided in a super-eco ship.
This electric propulsion system includes a rotating electric machine 1, a power supply unit 3 that supplies DC power, a power converting device 5 such as an inverter that converts the DC power supplied from the power supply unit 3 to AC power, and supplies the AC power to the rotating electric machine 1, a pushing propeller 6 that is connected directly to the rotating electric machine 1, and a field power supply 7 that supplies a field current to the rotating electric machine 1.
To achieve high efficiency, a synchronous motor formed with a rotating field winding la and a stationary armature winding 1b is used as the rotating electric machine 1, and the power supply unit 3 is normally formed with a power generator 3a and a power engine 3b connected to each other.
Unlike a conventional directly-connected propulsion machine that increases and reduces the output of a power engine so as to increase and reduce the propulsion power of the pushing propeller, the electric propulsion system shown in FIG. 5 operates the power engine on maximum output at all times (not shown in FIG. 5, see FIG. 6 instead), and increases and reduces the electric power to be supplied to the rotating electric machine 1 with the power converting device 5, so as to increase and reduce the propulsion power of the pushing propeller.
In this electric propulsion system, the output of the power engine is always kept at a fixed value. Accordingly, compared with a conventional directly-connected propulsion machine, this electric propulsion system can restrict the toxic substance emission such as carbon dioxide emission to a much smaller amount, and can reduce the adverse influence on environments.
However, unlike a conventional directly-conhected propulsion machine, this electric propulsion system uses a motor, a power generator, and a power converting device. Therefore, this electric propulsion system has lower transmission efficiency than a conventional directly-connected propulsion machine.
FIG. 6(a) is a schematic block diagram showing the transmission efficiency of the conventional electric propulsion system shown in FIG. 5. In FIG. 6(a), each of the values having the symbol Δ attached thereto represents a loss (%), and each of the values not having the symbol Δ attached thereto represents transmission efficiency (%). The numerical values shown on the upper side in FIG. 6(a) represent the transmission efficiency (%) at a rated speed, and the numerical values shown on the lower side in FIG. 6(a) represent the transmission efficiency (%) at a ½ vessel speed.
As shown in FIG. 6(a), the transmission efficiency of 79.7% at the rated speed, and the transmission efficiency of 63.3% at the ½ vessel speed are not sufficiently high, from a viewpoint of energy saving.
The low transmission efficiency is mainly due to the rotating electric machine serving as a power generator or a motor having low transmission efficiency.
Therefore, the transmission efficiency of the entire electric propulsion system is expected to become higher by increasing the transmission efficiency of the rotating electric machine serving as a power generator or a motor.
In a case where the rotating electric machine 1 of FIG. 5 is used as a wind-power generator, a load device 4 such as an industrial motor or a general electric power supply unit, instead of the power supply unit 3, is connected in the power unit 2. In this case, however, low-speed large torque is also required. Therefore, the rotating electric machine serving as a power generator or a motor is required to have higher transmission efficiency.
In a case where low-speed large torque is required as in a ship electric propulsion motor or a wind-power generator or the like, the rotating electric machine becomes a low-voltage, large-current rotating electric machine, having a small induced electromotive force.
Accordingly, in the conventional electric propulsion system shown in FIG. 5, the copper loss increases, and the transmission efficiency becomes much lower, as the armature current increases. Also, since the heat generation from the armature winding increases, it is necessary to make the refrigerating unit larger in size, which is disadvantageous.
To achieve higher transmission efficiency and easy refrigeration, the current density of the armature winding may be lowered. In such a case, however, the rotating electric machine becomes larger in size, and therefore, there is a limit to the decrease in the current density.
As a conventional technique, there has been an invention of a motor having higher outputs without an increase in power supply capacity. However, such an invention does not solve the problem of low transmission efficiency (see Japanese Patent Laid Open 2005-237175, for example).